Using bump node to achieve what displacement (on Bump mode) could do
If you wish to combine displacements you can do so with the MixRGB node here is a basic node tree combining a cobblestone and bricks displacement map together, although not using the bump node you can try using your bump maps in the image textures:



This produced the result:



Note that the MixRGB node in the diffuse color socket was not necessary, only in the displacement socket. This setup can be used regardless of the other shaders, specular workflow or using the principled shaders will not affect it as the displacement socket is separate from the other material outputs. So for your situation of using two principled shaders together this is a time and effort efficient way of getting nice and detailed combined displacement results from two materials.